Raspbian finally got the update from Jessie to Stretch and after upgrading not so important RasPis first and nothing bad happened. Today finally this very server got the upgrade as well.
The only problem I had was that MariaDB was not automatically updated from 10.0 to 10.1.
For some reason the
mariadb-client-10.0 blocked during upgrade but the rest of the MariaDB packages worked, so the database was not working.
The solution was simple
apt install mariadb-server mariadb-client
again and afterwards a 2nd reboot of the machine to get everythin working again.
Everything else was as simple as
apt update apt dist-upgrade
and answering some questions whele the machine got to work. It took some time, but was done within an hour.
Nevertheless, you should make a backup of the important data and config files before starting the upgrade process.